feat(M002): smart playlists — rule engine, editor UI, sidebar integration

Recovered from orphaned worktree commits (complete-milestone failed to merge).

Backend:
- Migration 9: is_smart + smart_rules columns on playlists table
- smartplaylist package: parameterized WHERE clause builder, field whitelist, genre subquery
- playlist.Service: Create/Update/Evaluate/Preview/GetRules smart playlist methods
- 65 tests (49 rule engine + 15 service + 1 migration)

Frontend:
- yj-combobox: reusable typeable dropdown with keyboard nav, ARIA, blur-race fix
- smart-playlist-editor: row-based rule builder with live preview
- smart-playlist-details: evaluate, refresh, play, shuffle, edit rules
- Sidebar: filter icon, Smart badge, create button, routing
- Queue snapshot on play/shuffle

backend/database/sql/sqlcgen/models.go

@@ -73,10 +73,12 @@ type PlayerState struct {
 }
 
 type Playlist struct {
-	ID        int64
-	Name      string
-	CreatedAt time.Time
-	UpdatedAt time.Time
+	ID         int64
+	Name       string
+	IsSmart    int64
+	SmartRules sql.NullString
+	CreatedAt  time.Time
+	UpdatedAt  time.Time
 }
 
 type PlaylistTrack struct {